There are two principal types of fuel pumping systems. One is a suction type fuel delivery system and the other is a pressure type fuel delivery system.
Suction type fuel pumps are the most common type of fuel delivery pump used outside of the United States. A suction pump is typically a positive displacement type pump housed inside the fueling dispenser. The fuel is drawn or sucked under negative pressure from the fuel storage tank through an underground piping system to a single fuel dispenser. For safety reasons, the fuel dispenser is often a substantial distance from the storage tank.
The advantage of a suction pumping system is that in the event of a breach in the fuel delivery line, all of the fuel in the line will drain back into the tank, and no fuel is pumped into the ground under pressure. There are several disadvantages of a suction piping system:
(1) It typically requires one pump per hose, or many pumps per fueling facility (typical is 12 suction pumps per fueling facility). Each pump requires its own piping run, which results in excessive piping and greater risk of an environmentally dangerous fuel leak through the additional pipe runs.
(2) Pumps are located inside the dispenser. This is an inconvenient and dangerous area to perform routine service work. Customers frequent the dispenser area and could be in danger or at unnecessary risk when the pumps are being serviced.
(3) Suction pumps commonly experience fuel vapor lock and can lose prime especially in warm temperatures or at high altitudes. When a pump loses prime, highly flammable fuel vapors are compressed and pumped through the system, increasing the chance of a dangerous explosion.
(4) Suction pump systems typically have lower flow rates than pressure type pumping systems and are not desirable for use at large fueling facilities with many fueling points.
Pressure pumping systems are more commonly used in the United States and Mexico. A pressure pump is commonly referred to as a submersible pump. The pump and electric motor are located inside the bottom of the fuel tank submerged in the fuel itself. The submersible pump is designed to pump fuel from the bottom of the fuel storage tank to one or more dispensers though an underground fuel delivery piping system. The advantages of submersible pressure type pump are as follows:
(1) Only one pressure pump is required per fuel grade (typical is 3 pumps per fueling facility).
(2) Pressure pumps require less underground delivery piping because the underground piping may be routed in series or a branching layout. Less underground piping saves money and reduces the risk of piping leaks.
(3) Pressure pumps are located at the tanks away and from the fueling customers. This is a more convenient and less dangerous area to perform routine service work.
(4) Pressure pumps located at the low end of the piping system cannot loose prime and are not affected by heat or high altitude.
(5) Pressure pumps systems typically pump a higher volume of fuel than a suction pump (they are more suitable larger high volume fueling facilities).
The disadvantages of a of submersible type pressure pumps are as follows:
(1) Although a submersible pump can pump a higher volume of fuel than that of a fuel suction pump it can only supply about 6 nozzles at one time or a maximum of 65 gallons per minute. Many large fueling facilities exceed the capabilities of submersible pumps when several nozzles are activated simultaneously.
(2) A submersible pump's electric motor is dangerously submerged in the fuel located inside the fuel storage tank. Electric power inside the tank increases the potential of an explosion especially when the tank is low (due to increased fuel vapors).
(3) Locating the pump/motor inside the tank means that a long pump column is required to be installed at the factory and not in the field. The result is that the submersible pump is awkward to handle and ship (can be up to 15 feet long), more costly to ship, and thus more likely to incur shipping damage during transit and while handling.
(4) The fuel flow path through a submersible pump is restrictive and creates considerable friction loss. The electric motor is directly in the flow path with only a tiny gap around the outside of the motor for the fuel to pass by.
(5) A submersible pump/motor is inserted into the tank typically through a 4″ tank fitting. Therefore the outside diameter of the pump/motor must be smaller than the inside diameter of the tank fitting. This requires submersible pumps to use high aspect ratio electric motors (long and thin motors) which are inefficient. In addition these submersible pumps have small diameter impellers (less than 3.5″ in diameter) that are not designed for high flow output.
(6) A submersible pump motor has a “dry stator”. This means that the motor's stator is contained within a sealed stainless steel metal casing. Stainless steel is a non-magnetic metal which becomes a restrictive barrier between the stator and rotor which operates on electrically generated magnetic power. The stainless steel casing reduces the efficiency of the submersible pump motor because it retains heat and interferes with the magnetic motor.